Volume 53, Nº 16 (2019)

The optical properties of dielectric nanostructures strongly depend on dielectric nanoparticles (NP) polarizability which can take the high values even interacting with low-intensity radiation. In this article, we are trying to find out if this is possible for electron transits to indirect exciton (IX) states at the NP surface. Because the exciton energy spectrum depends on NP shape and size, we research these factors influence on NP polarizability within the dipole approximation framework. For a quantitative assessment, we use a nanosystem containing a spherical dielectric alumina nanoparticle in a dielectric medium with a lower permittivity. We estimated the impact of NP size effect on the NP energy spectrum and the NP polarizability in case of exciton transitions to spatially separated electron and hole condition at the interface of dielectric nanoparticle and dielectric medium. Thus, using the variational method, we found that NP polarizability can achieve many orders of magnitude higher values than for atomic and molecular electron transitions case. It’s take place for infrared, visible and ultraviolet radiation ranges. Also was found that size-dependence of IE energy spectrum weakly affects on NP polarizability. The obtained results can be used for creating the nano- and heterostructures for advanced nanophotonic applications that operate in conditions of weak optical fields.

The low-temperature (T = 2 K) optical spectra (transmission, reflection and luminescence) of a compositionally graded CdS_1 – xSe_x/CdS heterostructure with a low Se content near the sample surface were studied. The Morse potential model of the near-surface exciton potential well is shown to be in a good agreement with the experimental data on the excitonic reflection and makes it possible to identify the radiative recombination channels of free and localized excitons in the structure under study.

The photoluminescence and excitation of luminescence spectra of a series of samples containing two CdTe layers D1 and D2 with a nominal thickness of 1.5 and 4 monolayers in a ZnTe matrix were studied. The samples differ in the width of the ZnTe spacer separating the D1 and D2 inserts and constituting 15, 25, 35, 45 and 55 monolayers (samples nos. 1–5, respectively). ZnTe layers are grown on a GaAs substrate by the standard molecular beam epitaxy mode, while CdTe inserts are grown in atomic layer epitaxy mode. It is shown that when the barrier thickness is less than 25 monolayers electronic states in the quantum wells formed by D1 and D2 layers are tunnel-coupled, and only one band is recorded in the emission spectrum, caused by the recombination of excitons in the deep quantum well. At larger thicknesses of the spacer, two bands I₁ and I₂ are observed in the luminescence spectrum, which are associated with the recombination of excitons in shallow (D1) and deep (D2) wells, respectively. It was found that the intensity ratio I₁/I₂ significantly depends on the energy and intensity of the excitation. Possible origin of these dependences are discussed.

Composites consisting of Ge nanoclusters embedded in GeO₂ matrix were modified by selective removal of the germanium dioxide in deionized water or HF. Thin (up to 200 nm) and thick (300−1500 nm) GeO₂{Ge-NCs} heterolayers were studied before and after the etching using Raman spectroscopy, scanning and spectral ellipsometry, scanning electron microscopy. It was found that a stable skeletal framework from agglomerated Ge nanoparticles (amorphous or crystalline) was formed after the etching of thin GeO₂{Ge-NCs} heterolayers. When removal the GeO₂ matrix from a thick GeO₂{Ge-NCs} heterolayer, released Ge nanoclusters were arranged in a vertically ordered chains.

Graphitic carbon nitride was synthesized via thermolysis of urea and then ultrasonic exfoliated from colloidal solution to obtain phase pure ultrafine powder of g-C₃N₄. It was shown that ultrasonic-assisted exfoliation of the initial graphitic carbon nitride powder leads to an increase in its phase purity (PXRD), a change in the morphology (SEM), a decrease in the band gap from 2.93 eV to 2.85 eV (DRS) and an increase in the specific surface from 58.6 m²/g to 136.7 m²/g (BET). In addition, it was found that the exfoliated g-C₃N₄ is an effective catalyst for the process of electrocatalytic reforming – the hydrogen evolution from the water-alcohol solution. Based on volamperometry, it was found that the hydrogen overpotential of graphitic carbon nitride is equal to 249 mV (at 10 mA/cm²), and the Taffel slope is 112 mV/dec. The results of cyclic voltammetry of the electrode based on exfoliated g-C₃N₄ indicate its high stability, which allows us to consider the exfoliated graphitic carbon nitride as a promising basis of materials for electrocatalytic reforming of alcohols.

The article presents the study of ZnO–Al₂O₃ thin coatings prepared by a polymer-salt method using the polyvinylpyrrolidone and zinc nitrate and tin (IV) chloride solutions. The materials were studied by spectroscopic methods and SEM analysis. It was found that prepared coatings have transparency in visible spectral range and demonstrate the ability to generate singlet oxygen under UV irradiation.

Supramolecular structures (domains, associates, nanoparticles) based on conjugated polymers are attractive basis for creating new materials for use in modern electronics. In this work we propose a mechanochemical method of synthesis of electrochemically active aniline- and N-(2-hydroxyethyl)aniline-based polymers. The effect of hydroxyethyl moieties on self-assembly of nanoparticles has been studied. Methods of dynamic light scattering and transmission electron microscopy have shown formation of nanosized particles of polyaniline and submicron particles of poly[(N-2-hydroxyethyl)aniline]. The electrical properties of materials were investigated: conductivity of polyaniline is 0.14 Sm/cm, which is constant over a wide range of temperatures and frequencies; conductivity of poly[(N-2-hydroxyethyl)aniline] is in range 2 × 10^–9–1 × 10^–6 Sm/cm, which strongly depends on temperature and alternating current frequency.

The work presents experimental data of Ga⁺ focused ion beam etching of disc and ring patterns in Si₃N₄/GaN structure. The reasons for the difference in etching depth between the discs and the rings are described.

The main features of self-catalyzed III–V nanowire growth according to the vapor-liquid-solid mechanism were analyzed using Monte Carlo simulation. The nanowire growth kinetics, flux ratio influence on the nanowire morphology and growth rate were considered. For some growth conditions, the self-equalization effect of metal drop sizes during the self-catalyzed III–V nanowire growth was demonstrated. It is revealed that, only under the adsorption growth mode, all drop sizes reach a single stationary value.

The silica glass substrates were implanted by ⁶⁴Zn⁺ ions with dose of 5 × 10¹⁶ cm⁻² and energy of 50 keV. During implantation the ion beam current density was less than 0.5 μA/cm² to avoid the substrate magnetically heating. After implantation, the substrates were subjected to isochronous for 1h heat treatment in oxygen atmosphere at temperature range from 400 up to 1000^oC with a step of 100^oC. Zn K-edge EXAFS spectra were measured in fluorescent mode. According to Zn K-edge EXAFS data, Zn in as implanted state and after oxidation at 800^oC is fully oxidized and surrounded by O atoms with coordination number of 4. However, in as implanted state there were disordered orientation of Zn atoms, and ordered tetrahedral coordination occurs only after heat treatment at 800^oC.

The effect of annealing temperature and time on the luminescence intensity of the InGaN/GaN heterostructure subjected to ion beam etching was studied. We show that annealing at a temperature of 1100°C makes it possible to eliminate the radiation defects in the GaN layers that arise in the etching process with a focused Ga⁺ ion beam (30 keV).

On the example of synthetic single crystal diamond with negatively charged nitrogen-vacancy center (NV)—center, we have experimentally reviled the changes in the polarized spectrum of the first-order Raman scattering due to the presence of nitrogen impurities. It has been found the appearance of rather intense spectra of the Brillouin zone-center optical phonons in an unpredictable crossed polarization of the incident and scattered light, while in the previously reported Raman data it was observed only in parallel polarization of the incident and scattered light. Such a recovery of Raman intensity is prescribed to the change of the local symmetry from the point group O_h to C_3v in accordance with the results of Raman scattering polarization rules based on conclusions of the group theory. The data make obvious also the observation of the strong in-coming resonance with the nitrogen NV ⁰ center electronic transition.

The recent advances in nanowire (NW) growth technology have made possible the growth of more complex structures such as core-multi-shell (CMS) NWs. We propose the approach for calculation of electron subbands in cylindrical CMS NWs within the simple effective mass approximation. Numerical results are presented for GaAs/Al_0.3Ga_0.7As radial heterostructure with AlGaAs-core and 4 alternate GaAs and AlGaAs shells. The influence of an effective mass difference in heterolayers is discussed.

We derive one-electron/hole probability density in ID semi-relativistic system within quantum field approach, considered as an alternative to Foldy-Wouthuysen transformation. We propose to consider single wall semiconductor carbon nanotube as the physical realization of such ID system. Application of Bogoluybov transformation to nanotube two-band Dirac-like k·p Hamiltonian predicts the additional smearing of position probability density as regards common quantum mechanical smearing, described by the square of wave function. It is shown, that under application of external potential, the revealed difference causes arising of non-local Darwin-like terms in the one-component Salpeter equations, describing separately positive and negative energy states. It is shown within proposed approach that the predicted peculiarity of defined probability density is due to unusual behavior of position operator in multi band k·p problem.

Nanosecond differential absorption measurements of spherical CdSe QDs at the quasi-stationary resonant and nonresonant excitation of basic exciton transitions are reported. The significant decrease in absorption and saturation of the basic exciton transition 1S_h3/2–1S_e are explained by the state filling effect. The observed blue shift of differential absorption minima with increasing pump intensities was explained by strong exciton–phonon interaction. In addition, the 2S_h3/2–1S_e exciton transition bleaching and induced absorption of mixed 1P_h3/2–1P_e and 2S_h1/2–1S_e exciton transitions were found.

CdS nanoparticles (NPs) as quantum dots (QDs) added into low density polyethylene (LDPE) can significantly change the MW dielectric properties of the composites. Introducing into the matrix of low density polyethylene CdS QDs increase the dielectric constant of more than twice. CdS QDs with concentration of 5 wt % to 20 wt % were prepared by the method of high-speed thermal decomposition. The size of CdS nanoparticles ranged from 2 nm to 7 nm. Experimental samples based on polymer-based nanocomposite in the form of thick films with a thickness of 80 to 100 microns were prepared by thermocompression. Complex dielectric permittivity of thick film samples was measured by T/R method from 2 to 8 GHz at room temperature (25°C). For samples with 20 wt % CdS NPs size effect was found. The change of dielectric properties of polymer composite materials is associated with the size of QDs. The dielectric constant and dielectric loss increases with decreasing size of the quantum dots. It is shown that the highest mean value of the dielectric constant and dielectric loss at a frequency range of 5 GHz to 8 GHz for cadmium sulfide nanoparticles with size of 4 nm were 12 and 189 dB/m, respectively. For QDs with ones of 6 nm average dielectric constant and dielectric loss were 5.8 and 134 dB/m, respectively. The effective dielectric constant and dielectric losses in the microwave range can be changed by CdS QDs of different sizes what consequently increases the possibility of using polymer nanocomposites as core elements of the microwave band-pass filters for various purposes.